1. Field of the Invention
The present invention relates to a method for manufacturing an electrostatic actuator that employs an electrostatic force, generated by a drive power source which applies a voltage between opposing electrodes, for displacing the opposing electrodes relative to one another. More specifically, the present invention relates to a method for forming a hydrophobic film on the surface of at least one of the two electrode members of the electrostatic actuator.
2. Description of the Related Art
Actuators with a microstructure formed using semiconductor microprocessing technologies are widely used in ink jet heads for ink jet printers. These microstructure actuators can be driven in various ways, one of which is electrostatic drive, a method that uses electrostatic force for drive power. Examples of ink jet heads that use electrostatic force to eject ink drops may be found in JP-A-5-50601 (1993), 6-71882 (1994) and EP-A-0 580 283.
This type of ink jet head has, in communication with each nozzle, a respective ink chamber whose bottom is formed as an elastically deformable diaphragm. The diaphragm is disposed opposite a substrate with a certain gap therebetween. Mutually opposing electrodes are also disposed on or by the diaphragm and substrate, respectively, and the space between the electrodes is sealed. In this case, the diaphragm and the substrate form the two opposing electrode members of the electrostatic actuator. When a voltage is applied to the electrodes, the electrostatic force created in the gap causes the bottom of the ink chamber, i.e., the diaphragm, to vibrate as a result of the electrostatic attraction to and repulsion from the substrate. The change in the internal pressure of the ink chamber resulting from this vibration of the ink chamber bottom causes one or more ink drops to be ejected from the ink nozzle. A so-called xe2x80x9cink-on-demandxe2x80x9d drive method whereby ink drops are ejected only when needed for recording can thus be achieved by controlling the voltage applied to the electrodes of the electrostatic actuator.
If moisture gets on the opposing surfaces of the opposing electrodes (i.e., on the bottom surface of the ink chamber and on the opposing surface of the opposing substrate) while the ink jet head is being driven by repeatedly applying a voltage to the electrodes, the charge of polar molecules may cause a drop in electrostatic attraction or repulsion properties. If polar molecules adhering to the opposing surfaces form hydrogen bonds, the bottom of the ink chamber (i.e., the diaphragm) may stick to the substrate and can become inoperable.
One possibility of avoiding these problems is to treat the opposing surfaces so that they are made hydrophobic. One means of achieving this is to coat these surfaces with an oriented monolayer of perfluordecanoic acid (PFDA).
An electrostatic actuator which is used for moving micro mirrors and in which PFDA is used for hydrophobic treatment is proposed, for example, in JP-A-7-13007 (1995) and in corresponding U.S. Pat. No. 5,331,454. These documents are directed to a method of preventing the opposing electrode surfaces of the actuator from sticking together when driven by forming an oriented monolayer of PFDA on these surfaces.
Hydrophobic processing using PFDA, however, leaves the following problems to be solved. First, the durability of the PFDA layers formed by simply depositing PFDA on the opposing surfaces of electrode members displaceable relative to one another is insufficient. Consequently, the PFDA layer separates from the surface of the underlying electrode members as a result of the electrostatic field being repeatedly generated between the electrode members to repeatedly displace them relative to each other. These separated layer particles then tend to clump together, creating foreign matter inhibiting relative displacement between the electrode members. When such foreign matter is formed, the danger of the electrostatic actuator becoming inoperable arises.
The gap between opposing electrode members in an electrostatic actuator is preferably as narrow as possible in order to generate a sufficiently high electrostatic force at a relatively low voltage. It is also preferable to minimize this gap as much as possible in order to reduce the size and to achieve a higher density arrangement of electrostatic actuators. PFDA molecules are relatively large, however, and if the gap becomes too narrow, it is not possible to deposit PFDA on the opposing surfaces separated by this narrow gap.
It has also been proposed to use a hexamethyldisilazane (HMDS) film to prevent relatively movable members in a microstructure from sticking together. However, such proposal does not provide any suggestion of sealing a gap in an electrostatic actuator using an HMDS film in the manner proposed by the present inventors.
Therefore, it is an object of the present invention to overcome the aforementioned problems. It is another object of the present invention to provide a method for manufacturing an electrostatic actuator having a durable hydrophobic film. It is yet a further object of the present invention to provide such a method that includes depositing a hydrophobic film on the surfaces of opposing electrode members, which are displaceable relative to each other by an electrostatic force, even when the gap between the opposing electrode members is narrow.
In accordance with embodiments of the invention, a method for manufacturing an electrostatic actuator including a first electrode having a first surface and a second electrode having a second surface opposing the first surface with a gap disposed therebetween, a driver for displacing the first and second electrodes relative to each other by producing an electrostatic force therebetween, and a hydrophobic film formed on at least one of the first and second surfaces. The method comprises the steps of: depositing a hydrophobic film on at least one of the first and second surfaces, the hydrophobic film being formed from a compound having the functional group R3xe2x80x94Sixe2x80x94X, where R is from the alkyl group and may be, for example, methyl or ethyl; and sealing airtight the gap between the first and second opposing surfaces so that the hydrophobic film is deposited stably on at least one of the first and second surfaces.
Generally, the hydrophobic film may be formed from an organosilicate compound having a hydrophobic functional group and the ability to react with a hydroxyl group. Specific compounds from which the film may be formed include hexamethyldisilazane (HMDS), hexaethyldisilazane, trimethylchlorosilane, triethylchlorosilane, trimethylaminosilane or triethylaminosilane. The preferred compound is HMDS.
This hydrophobic film, formed from a compound in accordance with the present invention, is more durable than a hydrophobic film of PFDA. Furthermore, molecules of such compound are small, and can therefore be deposited on one or both of the opposing surfaces even when the gap between them is narrow.
The inventors of the present invention investigated the durability of an HMDS hydrophobic film (HMDS film) formed on the opposing surfaces when the HMDS film was exposed to the air immediately after deposition. As shown in FIG. 7, it was found that durability drops sharply immediately after exposure, and then settles to a specific level after several minutes. If then left exposed for several days, durability gradually recovers. More specifically, when the gap between the opposing electrode members is sealed during period B in FIG. 7, and charging/discharging of the capacitor formed by the opposing electrode members is repeated four to five million times, a gelatinous substance (foreign matter) is formed in the gap between the opposing electrode members, and operating the actuator becomes difficult. The earlier this gap is sealed, however, the longer it takes for this gelatinous substance to appear (period A). More specifically, the greater the concentration of HMDS in the gap, the more difficult it becomes for this gelatinous substance to appear in the gap. On the other hand, creation of this gelatinous substance also becomes more difficult when the time until the gap is sealed with respect to the surrounding air exceeds a specific time (period C).
This unique phenomenon suggests that a surplus of HMDS in the gap facilitates the occurrence of this gelatinous substance as the charging/discharging of the electrostatic actuator is repeated, but that sealing an extreme surplus of HMDS in the gap conversely suppresses the occurrence of the gelatinous substance. In addition, if the delay until the gap is sealed exceeds a certain time, excess HMDS is eliminated by hydrolysis, and the surplus HMDS that is a source of foreign matter is thought to be eliminated.
These experimental results show that a durable hydrophobic film can be obtained after forming an HMDS film on the opposing surfaces by either (1) sealing the gap in which the HMDS is deposited while the HMDS concentration in the gap is still above a particular level, or (2) sealing the gap after leaving it exposed to the air for a plurality of days.
The present inventors conducted a further study with electrostatic actuators manufactured by method (1) above, that is, sealing the gap to air while the HMDS concentration therein was above a particular level. These studies confirmed that the durability of the hydrophobic film is improved to a level suitable for practical use if the gap is sealed with respect to the surrounding air while the HMDS concentration is 0.3% or greater. A hydrophobic film of sufficient practical durability can also be achieved when the gap is sealed while the HMDS concentration is 0.8% or greater. It was also confirmed that the sealing step can be performed at room temperature and atmospheric pressure.
The deposition step can also be achieved by simply exposing the opposing electrode members to an atmosphere of gasified HMDS at atmospheric pressure until a predefined concentration is obtained. After an HMDS film is thus formed, the gap between the opposing electrode members is sealed while they are kept in the HMDS atmosphere. By sealing the gap while in the HMDS atmosphere, the HMDS concentration in the gap can be reliably maintained above a specific level.
In further studies using electrostatic actuators manufactured by method (2) above, that is, sealing the gap after exposing the opposing electrode members to air for a plurality of days, it was found that moisture is preferably actively supplied during the exposure period. More specifically, leaving the opposing electrode members exposed to a moisture-rich atmosphere promotes HMDS hydrolysis, thereby more quickly eliminating the surplus HMDS that contributes to the production of foreign matter, and forming a stable hydrophobic film.
It should be noted that whether the gap between the opposing electrode members is sealed immediately or after a period of days in accordance with methods (1) and (2) above, a pretreatment step for reducing the moisture content in the gap preferably precedes the deposition step. More specifically, the manufacturing method for an electrostatic actuator according to the present invention preferably comprises a drying step for reducing the moisture content in the gap before the deposition step. This drying step helps stabilize the HMDS deposition, and can avoid variations in the HMDS deposition during the sealing step.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.